Impact of Detector Spatial Uniformity on the Measurement of Averaged LED Intensity

Photometers are widely used for the measurement of the average LED intensity as defined by CIE 127, under the circumstance that LEDs have many kinds of intensity spatial distributions. Significant measurement errors can be resulted due to the following facts: 1) a photometer with a diffuser generally has a spatial response that is stronger in the center of the detector; and 2) LED angular distributions of intensity often have a sidelobe about the central axis rather than a central peak. A special measuring facility for illuminance intensity distribution of LEDs and spatial response distribution of photometers has been designed. An evaluation factor “ c” has been introduced to describe the spatial response distribution uniformity of photometers. Three kinds of photometers with different spatial response distribution were used to measure the averaged LED intensity individually. Experiment shows that the photometer with a diffuser has a response distribution of a cosine function, and the illuminance intensity distribution of LEDs generally presents a sine, trapezoidal, or cosine function with the 6.5 ^° field of view for CIE Condition B. The results show that the measurement errors could be more than -5.10% when using the photometer with a diffuser to measure the average intensity of LEDs with a narrow beam angle. It is very important that the uniformity of spatial response distribution of the photometer should be considered when calibrating Averaged LED Intensity as it can introduce a significant error.